1. Field of the Invention
This invention relates to circuits for controlling lamps on an automobile and more specifically to circuits for sequentially turning on a set of lamps in order to signal a turn.
2. Description of the Prior Art
It is known in the art to use an array of lamps on an automobile to signal a turn. For example, on some automobiles, the turn indicators include a first array of three lamps to signal a left turn and a second array of three lamps to signal a right turn. The first and second arrays are typically mounted on the rear of the automobile. When signaling a left turn, the lamps in the first array flash in sequence, the innermost lamp turning on first followed by the adjacent lamp within the first array followed by the outermost lamp within the first array. This results in an impression of a moving signal in the direction of the intended turn of the automobile. The second array of lamps operates in a similar manner to signal a right turn.
There are a number of prior art circuits for controlling lamps providing turn signals of this nature. Such circuits are described in U.S. Pat. No. 3,500,312 issued to Donald Stankovich and U.S. Pat. No. 3,582,980 issued to Peter M. Marks. The Marks and Stankovich circuits include a bi-metallic switching element which periodically opens and closes. The switching element is coupled to a first lamp and a delay circuit, the delay circuit having a lead coupled to a second lamp and a lead coupled to a third lamp. When the bi-metallic switching element closes, the first, second and third lamps turn on in sequence. When the bi-metallic switching element opens, all the lamps turn off simultaneously.
Unfortunately, in a circuit of this type, there is a limit to the number of lamps which may be turned on in sequence because the sum of the delays between each lamp turning on must be less than the total on-time of the bi-metallic switching element.